Computed tomography (CT) imaging is a medical imaging procedure that utilizes X-rays and is used to generate diagnostic images of various body areas. Diagnostic CT images are obtained in the axial plane. Standard diagnostic images for clinical purposes are displayed in grayscale with various shades of black, gray and white. Grayscale CT images have been in use since commercialization of CT scanners in 1972. Pixel intensities are standardized and displayed according to the mean attenuation of the tissues in Hounsfield Units (HU). See Table 1. Each pixel represents a 3D volume of tissue known as a voxel. Radiodense contrast is frequently administered to patients via oral or intra-vascular routes to enhance detection of abnormalities.
Many, if not all, of diagnostic CT images are processed such that images can be displayed with slice thickness, field of view, smoothing kernel, windows, etc. so that imaging is optimized to the diagnostic clinical condition being evaluated. Windowing is a method for displaying a range of pixel intensities and is frequently used to look at a particular organ (e.g. brain vs. soft tissue vs. liver) or set of images for a specific imaging or clinical abnormality. Various forms of advanced processing and post-process of CT images are used to enhance diagnostic accuracy and decrease the complexity and time of interpretation of images by radiologists and other clinicians.
TABLE 1Approximate Attenuation of Various Tissues andSubstances in Hounsfield Units.Tissue/SubstanceAttenuation (HU)Air−1000 to −950 Fat−190 to −30 Fluid−10 to 20 White Matter25 to 35 Acute Ischemic Brain Tissue20 to −35Gray Matter35 to −45Normal Artery45 to −55Hyperdense Artery 55 to −100Acute Blood45 to 120Bone30 to 700Metal>1000
Color is used to enhance image diagnosis in some CT image applications (e.g. volumetric and perfusion imaging) but is not used in routine CT imaging. With the advent of multi-detector CT and development of volume-rendered post-processed images, color was introduced to enhance display of the 3D volume-rendered CT images. Additional advances in CT imaging (e.g. CT angiography, CT perfusion, hybrid imaging (PET CT and SPECT CT), and dual energy CT) have increased the usage of colors in post-processed CT imaging studies. The purpose of the color displays in most CT image applications is to either enhance viewing of 3D structures, to summarize large volumes of data in a series of images (e.g. CT perfusion imaging), or to overlay two different types of imaging for hybrid display (e.g. PET CT imaging).
Osteoporosis is a common, silent disease that is frequently underdiagnosed and undertreated until a fracture occurs. Osteoporosis contributes to 1.5 million annual fractures in the U.S., and its incidence and prevalence is growing. Approximately 1 in 2 postmenopausal women and 1 in 5 older men are at risk for an osteoporosis-related fracture, and approximately 10 million Americans have osteoporosis. There are significant healthcare and economic costs and a surprisingly high morbidity and mortality associated with osteoporotic fractures.
Screening is essential for the prevention of osteoporotic fractures but is currently underutilized, with more than half of osteoporotic fractures occurring in patients that were never screened. The most common method used to screen for osteoporosis is dual energy x-ray absorptiometry (DXA). DXA is a safe, reliable, noninvasive, relatively inexpensive x-ray method for measuring bone density of the lumbar spine or hips. The DXA device is small but often not available at most point-of-care facilities.
Opportunistic screening for abnormal bone density (osteoporosis and low bone density) using routine abdominal computed tomography (CT) images obtained for other purposes offers a potential solution to improve screening efforts. In 2014, there were approximately 81 million CT scans performed in the U.S. The majority (>60M) of these CT scans include images of the spine and contain useful information about bone density. Opportunistic screening using routine abdominal CT scans obtained for other clinical indications offers numerous advantages, as it requires no additional cost, patient time, scanner equipment, or radiation exposure. There are several potential methods for assessing spinal bone density on routine abdominal CT images for the purposes of opportunistic screening. An ideal opportunistic screening method for detecting abnormal bone density on routine abdominal CT images would be accurate, rapid, and have high inter-observer agreement.
Quantitative Computed Tomography (QCT) is a specialized procedure whereby spinal bone density is measured in reference to a phantom containing different concentrations of bone-simulating material. QCT provides an accurate measure of spinal bone density, with avoidance of osteophytes and other confounders, and has high inter-observer agreement. However, measurement of spinal bone density requires advanced post-processing software, and the processing of data is labor intensive and time consuming. By comparison, visual assessment of routine abdominal CT images to subjectively screen for abnormal bone density of the spine offers a potentially rapid method but is associated with poor accuracy and poor inter-observer agreement.
A recently proposed opportunistic screening method that utilizes routine abdominal CT images includes measurement of trabecular bone attenuation at L1 on a single sagittal reconstruction image. While this approach has been validated using DXA as a reference standard, this method has not been validated with QCT as a reference standard. Furthermore, a single attenuation measurement at L1 on reconstructed images with 3 mm section thickness may not accurately represent the true bone density at L1 or at other spinal levels due to the relative heterogeneity of bone density in the spine. Despite the fact that this method is relatively accurate and rapid, it is rarely used in clinical practice for the following reasons: it requires the user to remember to make the measurement when the purpose of the abdominal CT was for something different than bone density screening, it requires a number of mouse clicks and scrolling to prepare the image for measurement, and it requires additional time for interpretation of the quantitative information.
Because current methods for opportunistic bone density screening utilizing routine abdominal CT images have a number of pitfalls, there is a need for an accurate, rapid, and reproducible technique to screen for abnormal bone density on routine abdominal CT images that include the spine.